Stubborn internet worm Klez has topped the virus charts for the fourth month running and is now responsible for almost 97 per cent of user infections.

Antivirus software vendor Kaspersky Labs reported figures that consign previous troublemakers such as SirCam and BadTrans to the sidelines.

Klez was last week crowned the biggest virus of all time, ahead of SirCam, Melissa and I Love You, according to antivirus hosting company MessageLabs.

It found around 20,000 instances of Klez being recorded each day, affecting one in every 300 emails.

The worm deletes files on local and network drives and overwrites files with random data, making them impossible to restore.

The reasons for Klez's success have been put down to an ability to cover its tracks by using a random name from an infected machine's address book as the sender address, as well as using a large selection of subject lines.

Kaspersky Top 10 for May:

1 Klez 96.5%
2 CIH 0.8%
3 BadtransII 0.5%
4 Hybris 0.3%
5 PSW.Delf 0.2%
6 SirCam 0.2%
7 Elkern.c 0.2%
8 Macro.Word.Cap 0.1%
9 FunLove 0.1%
10 Magistr 0.1%